In contrast to the classical homogeneous atmospheric boundary layer, the urban boundary layer is more complex due to several specific features and processes. The main disturbing features of the urban environment are the buildings which introduce a large amount of vertical surfaces, high roughness elements, artificial materials, and impervious surfaces.

Building and ground-covering materials have radiative properties, such as albedo and emissivity, different from natural grounds and vegetation. In addition, the heat flux to or from the ground changes with surface materials. The anthropogenic energy use can be a significant fraction of the annual solar input and thus influence the local stability of air.

Three-dimensional ultrasonic anemometers, if representatively placed, provide a reliable estimate of atmospheric stability via the Monin-Obukhov length L or its inverse, 1/L, the Monin-Obukhov stability parameter. Measurements in Austrian cities or industialized areas conducted during the last years show that ultrasonic anemometers are able to detect characteristic features of urban stability, like an increased occurrence of unstable situations during the wintertime heating season. In contrast, classical discrete stability classification schemes based on cloudiness or net radiation data are not able to "see" such urban characteristics.

The presentation will demonstrate the ability of ultrasonic anemometers to determine especially the increased tendency to unstable situations in urban areas, compared to their rural surroundings and to the classical schemes to determine atmospheric stability.